Not a dumb question. Palram (Suntuff manufacturer) requires that you put the screws in the top of the corrugations in anything but vertical applications. This'll cut down on the possibility of leaks from cracks caused by over-torqueing the screws. Screws in a trough would act as water "dams" and be prone to leakage, where the screws on the peaks would shed water. You should also have the filler strips under the area being screwed to cushion the force of the screw. Polycarb WILL expand and contract with heat and cold, so it needs the cushioning to allow for that thermal movement. Those shoulda come with the panels for nothing.
Sure makes it hell to put a shade cloth on and off though...

Thanks Tom, I figured that was the answer but wanted to find out for sure. I purchase my polycarb at Lowes and you don't get a single thing with them, but you can purchase the white foam for insulation. I have never seen the black foam or wood forms to put under for structure.
Pam

Tom said it well: Leaks prevention and un obstructed flow channels.
You will need to buy the wooden base (to fit underneath) . You'll the screw those (every 24" or so) on the base first (pre drill) and then screw the board onto them.

I like the foam better because it's easier on the poly in case you have to crawl on it, it'll give. Home Depot gave me boxes of the foam, gratis, with my order of 5-1/2 squares of Suntuff. If you have one near you, check'em out...

I like the foam better because it's easier on the poly in case you have to crawl on it, it'll give. Home Depot gave me boxes of the foam, gratis, with my order of 5-1/2 squares of Suntuff. If you have one near you, check'em out.
Seysonn, Suntuff requires a screw at every corrugation for their warranty to be in effect. I just pre-drill, all the holes, using blue painters tape to prevent cracking, with a wood bit at moderate speed. A sacrificial block under the corrugation prevents splitting too. I tape the foam to the purlins where it should be and screw from the center. I usually seal with silicone at the joints, and along the ridge cap. Vertical apps don't require the foam or wood.

Seysonn, Suntuff requires a screw at every corrugation for their warranty
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Yeah, sure, Tom. I meant 24" ( or so) spacing LENGTH wise. So if your board is 6ft. long, you will need to screw down: toward the top, bottom and about 3-4 places in between.

Every corrugation? no way. but then I usually don't bother with warranties, too much trouble usually, and I've never had any problems in the last 8-10 yrs with the polycarb that I buy. I usually put the screws every couple waves.... and, honestly, all these years, I had never thought about how to apply, I just attached them in the troughs. I've never had any issues either.

hahaha, and no way will I ever crawl on it. I don't build enough structure in my roof for crawling around, I use minimum structure needed to hold up the polycarb. I've had my greenhouse over 20 yrs (the original 8' x 10') and have only had to rebuilt it once when it was over 15 yrs old. and that was mostly cos the first building had fiberglass on it and I hated the fiberglass.

Sheesh, I've bought 20 8x2 sheets of polycarb from Lowes and they never gave me a thing, except a receipt. ;-)

Pam

Seysonn,
I usually space the purlins at 2', which is the max for that material. It's the wind/snow loading that needs to be addressed.
Pam,
How do you put a ridge cap on without crawling on it? I built a platform out of 1/2" OSB that I screwed to the top beam so I could get it on. Kinda steep too. Had to put some battens on the OSB to keep from sliding off the 4/12 pitch.
Yeah, that fiberglass turns yellow after a few years.

well, I never had to bother with a ridge cap in the past, built is as a lean to, so... no ridge cap. We figured what we would try to do is put on the cap once one side is done, and try to put the ends on the other side under .... we'll see if that works.

Pam,
If you're using the polycarb ridge cap with the same kind of corrugations as the panels, you need to screw it down like the sheets.
Or, you can just buy lengths of galvanized steel ridge and screw it to the corrugations and use filler strips to plug the spaces in between. I've used a vented ridge cap on some applications too. It allows hot air to escape through non-leaking vents in the pieces.
Easy way to crawl on the structure is to cut a piece of 3/8" OSB to just short of where the ridge edge will go. Build a "stop" out of 2X that'll screw onto the top plate or exposed wall studs to hold the OSB. Screw that to the OSB. Ladder up to the platform and lay the pre-drilled cap in place. Come down and move the structure down the wall 4'. Don't forget to orient the seams away from the prevailing wind. Seal them with some silicone. Depending on the pitch of the roof, you may have to put some thin battens on the OSB sheet to keep from slipping off, or losing a drill...